The oil, gas and petrochemical industry desires to efficiently obtain hydrocarbons and process the hydrocarbons to produce desired products. Refining processes involve upgrading, converting or separating hydrocarbons (e.g., crude oil) into different streams, such as gases, light naphtha, heavy naphtha, kerosene, diesel, atmospheric gas oil, asphalt, petroleum coke and heavy hydrocarbons or fuel oil. Similarly, natural gas may be converted into industrial fuel gas, liquefied natural gas (LNG), ethane, propane, liquefied petroleum gas (LPG), and natural gas liquids (NGLs). The oil and gas processes are also often integrated with petrochemical systems to convert refinery streams into chemical products, such as ethylene, propylene or polyolefins.
To convert hydrocarbon feeds into petrochemical or basic chemicals, chemical conversion processes may be utilized. These processes typically involve using thermal or chemical reactors or furnaces to produce reactive hydrocarbon products, such as acetylene, ethylene or propylene in different proportions. As an example, steam cracking reactors are commonly utilized to convert the hydrocarbon feed into ethylene and acetylene, which may be further processed into various chemical products. The steam cracking reactors are utilized because they provide feed flexibility by being able to utilize gas (e.g., ethane) and liquid (e.g., naphtha) feeds.
Historically, the oil and gas refineries utilize the higher value distillates from the hydrocarbon feed, which are typically fungible fuels, such as mogas, natural gas and diesel. As a result, the petrochemical refineries utilize the remaining fractions, such as ethane, propane, naphtha and virgin gas oil, in their processes. However, few chemical conversion processes are able to directly employ natural gas or the lower value refinery feeds, such as aromatic gas oils or fuel oils. As such, there is a need for a process that can produce ethylene and acetylene from different feeds, such as advantaged feeds (e.g., natural gas and/or aromatic gas oils, for example).
To process these feeds, high-severity conditions (e.g., more severe operating conditions, such as higher temperatures) are generally involved to produce products having a higher value than the feed. High-severity conditions enable methane cracking and aromatic ring cracking, which do not occur at appreciable rates at typical low-severity conditions (e.g., conventional steam cracking conditions). At high-severity conditions, the primary products of thermal chemical conversion processes are acetylene and ethylene along with hydrogen (H2) and coke, which may vary in proportion depending on the temperatures, pressures, residence times and feed type utilized in the conversion process. Low-severity conditions may be still be used to convert higher hydrogen content refinery byproduct streams. At lower severity conditions, saturates may be converted to ethylene, propylene and butenes and alkyl aromatics may be converted to benzene, toluene and gasoline blend stock. Low-severity reactors operate above 700° C. to enable cracking or conversion to light olefins. Typically, low-severity reactors do not include lower temperature thermal processes, such as cokers or visbreakers, heat soakers, which do not produce substantial light olefins (≧10 wt % light olefin yield). The lower temperature thermal processes may typically operate at temperatures below 700° C. and even more commonly below 600° C. High-severity and low-severity conversion processes are typically based on different pyrolysis reactors, which may include pyrolysis alone or integrated with combustion chemistry. These reactors can be divided into eight different types: low-severity partial combustion, high-severity partial combustion, low-severity indirect combustion, high-severity indirect combustion, low-severity arc process, high-severity arc process, low-severity thermal pyrolysis and high-severity thermal pyrolysis. These pyrolysis reactor types differ in the means of generating and transferring the heat for the pyrolysis and/or in the severity utilized in the operating conditions. For simplicity, these differ types are discussed below as techniques, which include the low-severity and high-severity.
The first technique involves a partial combustion reactor. The partial combustion reactor burns part of the hydrocarbon feed to supply the heat to pyrolyse the remaining portion of the hydrocarbon feed. The partial combustion reactor includes pyrolysis chemistry (e.g., thermochemical decomposition of feed at elevated temperatures in the absence of oxygen) and combustion chemistry (i.e., exothermic chemical reactions between a fuel and an oxidant), with both chemistries occurring at the same time and with the products of both chemistries being an integral part of the reactor product. An example of this process is German Patent No. 875198 and U.S. Pat. Nos. 3,242,223 and 7,208,647. Specifically, U.S. Pat. No. 7,208,647 describes a partial combustion process that utilizes partial oxidation to convert methane into ethylene, while U.S. Pat. No. 3,242,223 describes a partial combustion process that utilizes partial oxidation to convert liquids into ethylene. Due to the nature of this process, however, an air separation plant is typically required and combustion products (e.g., carbon monoxide (CO) and carbon dioxide (CO2)) are significant components of reactor effluent that have to be managed. As a result, the partial combustion process has certain limitations, such as the requirement to remove the high levels of combustion products and associated processing or additional processing equipment.
The second technique involves an indirect combustion reactor. The indirect combustion reactor contacts a combustion product with the feed to be cracked in the reactor. As such, this process involves pyrolysis and combustion chemistry, but typically the combustion chemistry may occur at a different time or location and the pyrolysis chemistry, while occurring in the presence of combustion products, proceeds in a largely non-oxidative environment, resulting in the products of the two chemistries being an integral part of the reactor product. In a process used by Hoechst (High Temperature Pyrolysis) in the 1960s, the thermal energy from a hot combustion product is used to crack a feed in direct contact. Examples of these types of reactors are described in G.B. Patent No. 834419 and German Patent No. 1270537. As another example, the Kureha/UCC process is similar, except that the primary purpose of this process is to make ethylene. In this process, which is described generally in U.S. Pat. No. 3,419,632, the hydrocarbon feed is a crude oil or a distillate having a boiling point less than (<) 1050° C. Further, U.S. Pat. No. 7,208,647 describes an indirect combustion process, which directly contacts the combustion gas with the feed to be cracked. Similar to the discussion for the partial oxidation process, this approach suffers from the same limitations of having to have an air separation plant and manage the combustion products. Accordingly, this type of reactor and associated process also requires an expensive active quench step to stop the pyrolysis chemistry (e.g., water or oil).
The third technique involves an arc reactor, which includes plasma arc reactors and electric arc reactors. This process typically involves only pyrolysis chemistry. Arc reactors are commercially limited and typically operated in a few small plants and described in U.S. Pat. No. 1,860,624. This process involving this type of reactor typically uses a water absorption process for recovery of acetylene, which was initially developed in the 1940s. The electric arc process utilizes electric power to heat a feed. As an example, U.S. Pat. No. 7,119,240 describes an electric arc reactor and process. The drawback of the arc process is the high cost of utilities, such as electricity, required to generate the “arc” or plasma. As a result, this process is limited to small units integrated with supplies of “cheap” electricity, such as a hydroelectric plants or nuclear facilities.
The fourth technique involves a thermal pyrolysis reactor. Thermal pyrolysis reactors involve heating a solid material (e.g., by combustion) and using the heated solid material to provide heat to crack the pyrolysis feed (e.g., via pyrolysis chemistry alone). In the thermal pyrolysis processes, the combustion products are typically maintained separate from the pyrolysis hydrocarbon products or effluent. This pyrolysis technique involves various different types of reactors, such as a furnace (e.g., as used in steam cracking), a regenerative reactor (e.g., as used in the Wulff process) and others. For instance, thermal cracking is generally described in various references, such as U.S. Pat. Nos. 7,138,047 and 7,119,240. U.S. Pat. No. 7,119,240 describes an exemplary process for the conversion of natural gas into ethylene. In this process, natural gas is cracked in a furnace, actively quenched, and processed in a reactor to produce ethylene. As another example, U.S. Pat. No. 7,138,047 describes another steam cracking process that mixes a hydrocarbon feed with a dilution steam, flashing the mixture, and vaporizing a portion of the mixture in a pyrolysis reactor. In the process, the pyrolysis feed is passed through tubes in the radiant section of a pyrolysis reactor to crack the pyrolysis feed without contaminating it with combustion products. However, due to the nature of a tubular (metal) furnace, steam cracking is limited to effective cracking temperatures of below 1000° C. and residence times of≧100 milliseconds (ms), which do not allow conversion of either methane or aromatics, thereby limiting the feedstock selection. In addition, energy or furnace heat not used in cracking is partially lost in the furnace flue gas or in the quench, as products are quickly cooled to stop undesired reactions.
The “Wulff” reactor, as described in the IHS, SRI Consulting's Process Economics Program “Acetylene” Report Number 16 (1966) and 16A (1982) along with U.S. Pat. Nos. 2,319,679; 2,678,339; 2,692,819; 3,024,094, and 3,093,697, uses a reverse-flow pyrolysis reactor, which is typically operated at temperatures of<1400° C., to produce olefins and alkynes, such as acetylene. The pyrolysis feed is heated by refractories which have previously been heated by combustion reactions. The pyrolysis feed is cracked, and then cooled outside of the reactor. The relatively slow quenching is a characteristic of the Wulff process that leads to coke and soot formation from using inefficient indirect heat transfer (e.g., from checker brick). Coke formation in the reactor provides fuel during the combustion cycle and excess coke or soot may be alleviated by using a light feed, i.e., a hydrocarbon containing a high proportion of hydrogen. However, because the indirect heat transfer limits the rate of heat input in the Wulff process, certain pyrolysis feeds, such as methane, may not be economically processed, which limits the feed flexibility for this process. As a result, these reactors typically have limitations, such as poor heat transfer and greater soot generation resulting in poorer selectivity to desired products.
While the prior art describes using different pyrolysis reactors, these reactors described include various limitations, which reduce the efficiency of the process. For example, steam cracking is efficient in converting naphtha, but not efficient in converting methane. Likewise, certain high temperature pyrolysis techniques are more effective in converting methane, but too expensive to effectively convert naphtha. Accordingly, it is desirable to provide a process that converts hydrocarbon feeds into olefins, such as ethylene, in an enhanced manner with different reactors types to efficiently convert a broader range of feed molecules. In particular, it is desirable to provide a configuration that provides flexibility in the hydrocarbon feed utilized for olefin recovery. Accordingly, various combinations of different pyrolysis reactors are envisioned, where each type of pyrolysis reactor may efficiently crack a preferred portion of a hydrocarbon feed, which are described further below. These pyrolysis reactors may be coupled together with each of the reactors being associated with a different portion of the hydrocarbon feed.